1. Field of the Invention
The present invention is directed to the field of injection molding and, more particularly, to an improved method and apparatus for controlling the quantity of a moldable material introduced into the mold cavity by monitoring the area under the part-line separation curve.
2. Description of the Prior Art
The production of consistent and uniform products by the injection molding process has been a long-standing objective in the injection molding art. This objective has become relatively difficult to achieve as more and more injection molding products are produced which are increasingly complex with stricter tolerances. Furthermore, the trend towards fewer and fewer operators monitoring the injection molding processes and apparatus requires more precision automatic control of the process and apparatus.
The injection molding process involves a variety of interrelated parameters present in the injection molding material, apparatus, and control systems. Among these parameters are the type of material being molded, the consistency of the plastic and its characteristics, the molding cycle time, the machine shot size and/or weight, melt viscosity and temperature consistency, mold clamp pressure and injection pressure. As each of these parameters varies during the operation of an injection molding process the product uniformity may suffer without constant operator attention.
Various techniques have been utilized to determine which parameters should be monitored more closely to yield a more consistent molded structure and provide a more reliable injection molding process and apparatus. In the injection molding art, it is necessary to precisely control the amount of moldable material entering the mold cavity during the injection, or filling, phase of the injection molding process.
U.S. Pat. Nos. 2,433,132, 3,976,415 and French Patent No. 2,527,976 teach measuring the part-line separation in the mold elements to determine when to change the molding process from the injection phase to a pulsing of an injection ram to maintain a constant part-line separation during the curing of the mold. However, it has not been feasible to attain high precision accuracy with this technique.
U.S. Pat. Nos. 2,671,247 and 3,859,400 teach monitoring the pressure within the mold itself to determine when to terminate the injection of molding material. However, monitoring pressure alone does not result in a molded product of the desired precision because no compensation is made for variations in other parameters not taken into account U.S. Pat. No. 3,940,465 teaches measuring the part-line separation of the mold to control the cure time of the molding cycle. Similarly, this technique also fails to reflect all of the other variables which must be accommodated and taken into account to accurately control the molded part weight and its dimensions.
U.S. Pat. No. 4,135,873 teaches the measurement of the part-line separation and comparing the separation with a predetermined value and thereafter varying the injection pattern of the injection ram during the following molding cycle. This system does not provide control of the process on a real time basis, reflecting system conditions that are affecting the current cycle. Such a system merely reflects what occurred on the previous cycle, resulting in a tendency for the system to hunt rather than zero into a mode of operation which provides product consistency.
U.S. Pat. No. 4,131,596 teaches the measurement of the part-line separation to reduce the mold clamping pressure upon the measurement of a predetermined separation to minimize any damage to the mold due to flashing of the material at the part-line. This, of course, does not contribute to the control of product weight and dimension.
Japanese Patent Publication No. 11974 of 1978 discloses a method of controlling an injection molding machine wherein the part-line separation is measured and, upon reaching a predetermined reference separation, the machine is switched from a material filling mode to a dwelling mode. The mold separation is then measured and the maximum separation is determined. Thereafter, pressure during the dwell or curing phase of the mold cycle is controlled dependent upon the maximum separation reached to control the final mold separation value at the end of the cure time. Thereafter, the reference separation value for the switch point for the following cycle is changed to accommodate the variations in the machine operation detected during the first cycle. This system of control has the disadvantage that the switch point is determined by the preceding cycle and thus does not reflect the conditions of the current cycle. This system of control thereafter attempts to adapt to the variations in the molding conditions existing during the current cycle by controlling the holding pressure during the cure phase of the cycle which can adversely affect part weight and density uniformity.
U.S. Pat. No. 4,146,601 teaches the method of integrating the hydraulic pressure with respect to time until the cavity pressure reaches a predetermined value. The integrated value is compared with a predetermined value and an adjustment is made in the temperature and viscosity of the material in subsequent molding cycles. This technique is very complicated and information is based upon a prior cycle, not the current injection cycle. As a result, precision and accuracy are compromised.
U.S. Pat. Nos. 4,767,300 and 4,767,519 disclose a method and apparatus for control of an injection molding process based upon the part-line movement of the two faces of the injection mold. Although this system provides a control system which is reasonably effective within a limited range of perturbation in the operating parameters, its effectiveness quickly diminishes as that range is exceeded. This is due primarily to the fact that the relationship between part-line distance and the volume or weight of the moldable material being injected is not linear.
It is apparent from the foregoing that existing control systems tend to be overly complicated, inaccurate and incapable of making same-cycle corrections. Moreover, they generally require almost constant operator attention.
It is therefore an object of this invention to provide a method and an apparatus for precisely controlling the volume of moldable material injected into a mold during a molding process.
It is a further object of the invention to provide a method and an apparatus of the type stated which permits precise control in spite of relatively large perturbations in operating parameters.
It is a further object of this invention to provide a method and an apparatus of the type stated requiring relatively little monitoring by an operator.
Finally, it is an object of this invention to provide a method and an apparatus of the type stated which exercises control over the molding operation on a real time or same-cycle basis. In other words, data gathered and computed during a given molding cycle is used to control that very same cycle rather than subsequent cycles.